SAN FRANCISCO — Police issued arrest warrants Monday for two men accused of a New Year’s Eve attack on members of an all-male a capella group from Yale University.

Richard Aicardi and Brian Dwyer are charged with assaulting two members of the Baker’s Dozen outside a party held in honor of the 16 student singers. Witnesses at the time said the trouble started after the vocalists, wearing sports jackets and ties, sang “The Star Spangled Banner.”

Evan Gogel, one of the two most seriously injured chorus members, suffered a concussion when Aicardi, Dwyer and others repeatedly kicked him while he was on the ground, police said. Aicardi also was charged with punching Baker’s Dozen member William Bailey as he tried to get away.

“This was a cowardly attack on defenseless victims,” District Attorney Kamala Harris said. “We are going to hold accountable those who have been identified as responsible.”

In the 10 weeks since the widely publicized attack, the San Francisco Police Department has weathered criticism from families of the Yale students who were upset by what they considered a slow and inadequate response.

Police and Harris said the investigation was hampered because the young men could not identify their attackers. San Francisco investigators subsequently flew to Los Angeles and to New Haven, Conn., to gather more information.

Harris said Monday she did not have enough evidence to charge anyone in the assault on a Baker’s Dozen singer who suffered a broken jaw during the melee but said she had not ruled out the possibility that additional charges would be filed.

“If additional evidence emerges, we will move forward in that case as well,” said Harris.

"There is a general recognition that we don't need these military-style weapons in New Zealand, so it's very easy to win cross-party support for this," said Mark Mitchell, who was defense minister in the previous, center-right government and who supports the ban initiated by the center-left-led Labour Party.